Bob witnesses a murder, so reports the crime to the police. Later, lawyers representing the state/jurisdiction come to Bob, asking him to provide witness testimony. But Bob values his time, and doesn't want to spend hours at the court out of the goodness of his heart. Is Bob allowed to ask for monetary compensation to serve as a witness for the crime? Or does that create a conflict of interest ("I paid this witness $1000, now listen to them support my argument")?
A witness who is under a subpoena cannot condition their appearance or testimony upon being compensated.
Usually, a "lay witness" who is not offering expert testimony and instead just testifying about their personal knowledge of the facts of the case, is entitled to a statutory witness fee when appearing pursuant to a subpoena to be paid by the lawyer or law firm or government agency issuing the subpoena. But, this minimal amount is basically only enough to pay for mileage to get to the court house, parking, and perhaps lunch. It is even more minimal than the tiny compensation paid to jurors.
It is not unethical for an attorney to pay reasonable actual expenses of testifying to a lay witness to encourage them to cooperate with the process and as a courtesy to avoid inconveniencing them beyond the statutory witness fee.
For example, paying the cost of a hotel and dinner the night before if it is a long trip to court so that the witness doesn't have to leave home to go to court at 3 a.m. would not be unethical, even if it exceeds the statutory witness fee. But, it generally isn't proper to pay a lay witness simply to testify.
As an example, the statutory witness fees for federal court witnesses in Denver, Colorado, which is one of the most generous sets of witness fees, in part, due to the high cost of living there (state courts are usually less) is as follows:
Witness fee per day $40.00
Witness mileage, round-trip (per mile) $0.655
Witness subsistence reimbursement (Denver) (seasonal, Nov. 1 - Dec. 31, 2022 = $232; Jan. 1, 2023 - March 31, 2023 = $241.00;) for an overnight stay $278.00
Witness subsistence reimbursement (Denver) for last day of travel
(See General Services Administration per diem website page for seasonal changes [also lists of per diem rates for other Colorado locations], and GSA mileage website page.) $79.00
An "expert witness" testifying on matters of opinion arising from their expertise unrelated to the facts of a particular court case, is entitled to reasonable compensation and normally would testify voluntarily when not under subpoena, pursuant to a retention agreement upon which the expert may be cross-examined at trial to show a potential source of bias.
Sometimes a "non-retained" expert witness will testify under subpoena rather than voluntarily (e.g. someone who performed an autopsy on a body in a murder case) because they have knowledge of facts from their personal knowledge that are relevant in addition to having that knowledge enhanced by their professional expertise. Such witnesses generally are compensated at a reasonable rate for their services, but the analysis is different and is governed by the applicable civil procedure statutes and rules in the place where the trial is conducted.
Expert witnesses routinely receive compensation for their professional analysis, report-writing, and testimony at trial. The courts do not compel an expert to perform an analysis of some pertinent facts. The courts do compel eye witnesses to testify. This provides incentive to the witness to do as they were told by the court.
An attorney cannot pay a witness to provide their eyewitness testimony. However, it is never illegal to ask. Therefore, the simple answer is "asking questions" is legal. There is a federal law against bribing witnesses, 18 USC 201 which punishes one who
directly or indirectly, corruptly gives, offers, or promises anything of value to any person, or offers or promises such person to give anything of value to any other person or entity, with intent to influence the testimony under oath or affirmation of such first-mentioned person as a witness upon a trial, hearing, or other proceeding, before any court, any committee of either House or both Houses of Congress, or any agency, commission, or officer authorized by the laws of the United States to hear evidence or take testimony, or with intent to influence such person to absent himself therefrom
and state analogs. This is about paying a witness for their testimony, and not a blanket prohibition against paying a witness: here's a brief note on navigating the unclear waters about paying. For instance, the witness can be compensated for travel expenses, because the compensation is not for the testimony, it is for the travel that makes the testimony possible.
Bob is certainly allowed to ask for money in exchange for his testimony, but that doesn't mean he is entitled to receive that money or to skip the trial if he doesn't get it.
A witness like Bob will likely be subpoenaed to appear at trial. This means that it doesn't matter how much he thinks his time is worth; he will almost certainly have to show up to testify.
However, Bob will also likely be paid for appearing as a matter of course. Most witnesses are entitled to some payment for each day they spend in court, and they may also be entitled to compensation for traveling to the court. This fee will likely be much less than Bob thinks his time is worth, but it is generally not negotiable. In federal court, for instance, 28 U.S. Code § 1821 establishes the fees to which a subpoenaed witness is entitled, which will generally be $40 per day plus travel expenses. The fee schedule for appearance in state courts will vary from state to state.
I don't know that the prosecuting attorney is prohibited from paying Bob still more money to appear, but doing so would certainly be unusual and set off alarm bells. Further, the prosecutor would likely be required to disclose those payments so the defense could use them to suggest to the jury that Bob's testimony is less credible
Bob would be classed as an Ordinary Witness and can claim expenses etc (below) payable the Crown Prosecution Service (CPS) which is:
responsible for paying allowances and expenses to witnesses who are called to give evidence in prosecutions conducted by the Service.
An ‘ordinary witness’ is:
a witness to fact - a witness who is neither a professional nor expert and who gives evidence based on their first-hand knowledge of events relevant to the case.
Ordinary witnesses may receive compensation towards:
Money spent on refreshments and meals
Financial loss (e.g. loss of earnings)
*Other expense incurred (e.g. childcare)
There are limits on the amounts payable:
Bus and train fares (standard fare) should normally repaid in full
Travel by motor vehicle is paid at a rate per mile
Childcare will normally be reimbursed within a maximum daily amount. However, discretion should be applied where childcare cost exceeds the rate. An official receipt from a registered child carer must be provided.
Taxi fares are allowed in exceptional circumstances. Discretion should be applied for incidents such as:
Where this was the only form of transport available
If a witness is registered disabled or infirm.
If the witness is vulnerable
This should normally be agreed in advance and receipts must be provided.
Set limits apply to refreshments and meals
Maximum daily amounts apply to loss of earnings and other financial losses
The CPS will normally arrange the hotel accommodation for ordinary witnesses and pay for it. The overnight allowance covers an absence of 24 hours. Where the witness has had to arrange their own overnight accommodation the CPS will reimburse actual rates up to the maximum overnight rate, where receipts are provided.
Where a meal is also included, ordinary witnesses will be entitled to a fixed Personal Incidental Allowance. If no evening meal is included ordinary witnesses are additionally entitled to a Night Subsistence Allowance. This is a fixed amount towards the cost of meals, refreshments and other expenses where a witness has had to stay away from home.
Witnesses who stay with family or friends in place of the above will receive a single fixed payment. See ‘Current rates’ for Ordinary and Professional witnesses.