I'm in a dinner theater play like that right now. I was originally a minor character, so it didn't annoy me TOO much, but two weeks go one of the leads had to drop out so I was recast. Lordy, that's some bad dialogue they've got me speaking. Clunky monologues, horrible resolution (murder/mystery with no real mystery)... The director's not a purist, so I have some leeway on what I say, but I still need to hit some words just for the other actor's cues.

I'm doing the play as written, for the most part. I'm intentionally dropping some of the worst parts (and getting away with it) and rephrasing others, but trying to stay true to the script. It's not a classic, and couldn't be even with a major rewrite, but that's not my job. I'm just supposed to do it as well as I can.

My family has heard plenty of my complaints, but I don't plan on telling the director. I'm sure she's of the same opinion, but it's a dinner theater and this is the play that fit the venue. Not much we can do about it, and opening night is Friday so...

My take on it, RT:
You're helping out someone in your theater community who is in a pinch; if I'm reading you right it's just a one shot deal. You're getting a chance to act during a period of time you wouldn't have been anyway.  It might not be a good script but at least he's out there trying.  And it's just three weeks away. If it was me I'd find something nice to say and just do it mostly as it is....maybe some suggested changes here or there but not major rewrites of dialogue at this point.

Exactly how I approached it, Thudster.   Had the first read through tonight. Managed to make a few changes here and there,  but mostly kept it in tact.

In response to the question, would you do a play you didn't like, my answer would generally be no.  Don't direct shows you don't like--you'll turn them into something that contradicts their authors' intent.  Don't act in shows you don't like--lack of enthusiasm rubs off, and creates an unpleasant environment for all.  Let someone else, who does "get" it, do it.

However, in the context of this situation, you're helping a friend, and it sounds like a low-level commitment.  So, maybe you should do it and do it well.

I've acted in enough "original" plays that I've discovered something: they can, indeed, be very bad, but not infrequently, they're better than you think.  Many of the critiques we can level against plays because we have them in our hands become less consequential in the context of performance.  An audience might not notice the plot or character inconsistencies we do, if they're minor enough or we're compelling enough, and our attempts to make dialogue real for an audience and ourselves may capture a rhythm that's not intuitive as we read the script silently.

I'm not saying the play is necessarily good (I'll trust you that it's not), but it's probably at least playable.  Have a little faith that there are line readings that work, if only you discover them, and consider the experience a challenge that teaches you something else about acting (a process where we're perpetual students).

Good observations everyone.