I am a playwright working on a new play related to Ag-Gag laws. My plot involves a woman who is trespassing with another animal rights activist to film animal cruelty at a CAFO. Unbeknownst to her, the "activist" is actually an undercover policeman who has infiltrated the animal rights group. Once the woman has captured the abuse on her camera, I would like the undercover guy to reveal himself and arrest her. But is this at all realistic? (I don't mind a little poetic license, but I don't want to make it completely far-fetched.) Thanks for any insights!
A lawyer would have several questions here: is trespass an arrestable offence in this jurisdiction? Did the officer persuade the woman to commit the trespass (entrapment)? Do citizen's arrests and warrantless arrests exist here? It sounds, though, as if the level of realism you are looking for is "her lawyer turned up and a few hours later she was out on bail" or thereabouts: for that purpose, your scenario is fine.
As Justin pointed out, though, it's somewhat implausible to have an undercover operation revealed simply to make an arrest for trespass; whether she has taken photographs is irrelevant legally, so the time and resources (including the officer's cover) have been expended simply to make a misdemanour arrest that could have been made by a passing patrol, or even a security guard.
Realistically, if a bust was going to be made, uniformed cops will make the arrest of the entire group, or all who were in attendance, including the undercover officer who will be separated from the rest of the group to be debriefed. This is to keep the fact that the undercover officer may still need to be undercover as the lawyer may be able to pass word from the arrested activist to the rest of the group (cops cannot listen to lawyer's conversations with their clients... if the arresting officer is undercover, than she could warn others about the undercover officer and stop further investigation of the group.