Sometimes a voice actor will look at a script and think to themselves that something may sound better rephrased or expanded upon. But should they give in to this impulse? Is there ever a time when ad-libbing is acceptable? While we're discussing that, let's also touch on why the script-writer may or may not want to encourage ad-libbing.
In many forms of acting, it's often acceptable for the actor to do a little improv, add an extra line, change something up on the fly. And, there are certainly times in voice acting when it's acceptable or even encouraged. There are, however, many times when it absolutely is not. One obvious time you can't deviate from the script is when you're voicing something like an audiobook. Try to add in some funny extra dialogue quips, or even attempt to correct the author's grammar by rephrasing something, and you're just going to have to retake that part and do it as-written.
A lot of scripts outside of audiobooks, however, will still be considered set and unchangeable. Unless you have also been hired to do some copy editing, it's best to avoid making drastic changes without prior permission. That isn't to say that if you encounter a line where a word was obviously left out, that you can't record the line with the word that should have been there. But it means you really shouldn't take liberties with the script and change entire phrases, leave things out, or add lines in.
Sometimes there will be notes in the script which give permission to ad-lib in certain parts. Sometimes you will have a director either present or patched in, and you can ask them then and there if it's allowable to change things up. They may even ask you to try a line a few different ways, because listening to you deliver it the way it's written sparks a new idea for them. But if there are no such script notes, and the director isn't present/patched in, it's safest to just stick to the script as-is.
Now, if you're the writer/director of the project, why might you even want your actor to make such improvisational changes? There could be a few reasons. Perhaps the script is not written in your first language, but it is the talent's first language and they can better tell when phrasing is off. In that case, you might give them permission ahead of time to make minor adjustments for that, so long as they keep the meaning and tone. Most commonly, however, the benefit of ad-libbing is to provide a bit more humanity to something. It might just be some little comments the talent makes during a training video, where they break the fourth wall and bring the audience into some joke. Or maybe it's for a character in a game or animation, and you want reactions to feel real. It could be an ad where they're voicing a kid who's excited about a new theme park ride, and you want them to do a take as it's written, but are also open for them to give a few more reactions as the role they're playing.
Much like how talent should check with the client before any ad-libbing, the client may want to check with the talent if they're comfortable with doing a lot of improv (if more than just a smattering of it is required). If seeking talent through an audition, make sure to include it in the listing. If using a service or agency like FlyVoiceovers, you should be able to ask the agent for good recommendations on talent that can handle that level of improvisation. Again, if it's just a light bit of improv or ad-libbing desired, then there's no need to make these audition notes/requests (but DO still include notes in the script itself). It is always best, however, to have a script (even if you don't want the talent to fully stick to it), so that rates can accurately be determined.