It was a well-timed question, because I wasn’t entirely sure. There were two projects on the radar screen, both of them rewrites. I had a week to decide whether to do either.

The first was a difficult-but-potentially-great bio-pic about a major figure of the 1970’s. Everyone and their brother had tried to make the movie, but it had never gotten to the starting line. But there seemed to be new traction, along with a new (and high-profile) director who seemed up for the challenge.

The other was a high-concept comedy about a guy who hates musicals, who wakes up one day to find himself trapped inside one. It too had a well-chosen director, along with a studio that was very eager to make it.

I described both projects to various classes and listened for their reaction. For the bio-pic, I got respectful nods.

For “Bob: The Musical,” I got a laugh-and-or-chuckle, almost every time.

I think that’s because it’s really easy to see why the movie would be funny. In five seconds, you can visualize the trailer, the TV spots, and the one-sheet. You can hear what the star would say on Leno: “It’s a movie for people who love musicals, and especially for people who hate them.”

But the fact that Bob is obviously a movie doesn’t mean it’s obviously the movie I should write next. Here was my decision making process:

1. Which movie is more likely to get made?
Remember, as the screenwriter, I don’t get to decide which of the movies I write actually gets made. For that, I’m beholden to a hundred other factors, most of them out of my control. So if I’m going to dedicate months of my time to a project, it makes sense to pick one that will get made. So for that, I’d say Bob. It’s easy to make, easy to market. The bio-pic, on the other hand, has been in development for more than a decade. My script could be just one more sitting on the shelf.

2. Which movie will be better for my career-slash-reputation?
Tougher call. If I wrote a kick-ass version of the bio-pic, and if the director did a great job, and if the film got a terrific critical response, then I think that would be the winner. Notice: that’s a lot of “ifs.” But by the same token, if the film didn’t work (a “noble failure” in industry parlance), I’m not sure it would hurt me that much.

Even if I wrote the superlative version of Bob: The Musical, I wouldn’t clear any space on the mantel for awards. It’s just not that kind of movie. And in disaster, I’m not sure the movie would do me much harm either.

3. Which is the more challenging?
The bio-pic, no question. I’d be working out of my comfort zone, most notably in the time period. I was born in 1970, so trying to write about the adults of that era is difficult. In many ways, I’d feel more comfortable writing about cavemen, because at least that way I’d know that no former hippie was going to come up to me and say, “Man, you totally missed what it felt like to be there.”

By the way: challenging is good. All things being equal, I’d rather work on the challenging project than the one I could write easily. But challenging work takes longer, and forces me to ask the question…

4. Which do I have time to do?
This was really the deciding factor. Because of prior commitments, I have a limited window in which to do this next project. So whichever rewrite I choose, when I’m done with it, I need to be able to walk away. That was looking unlikely with the bio-pic, given the director’s schedule and work history. Also, knowing myself, I would probably choose to stay much more involved with something I’d struggled harder to write.

So, after meeting with the director, producer and studio folks, I signed on yesterday to rewrite Bob: The Musical. The original screenplay was written by Mike Bender, who is now adapting a Spanish film series over at New Line. Considering that his brother is one of the producers on Bob, I suspect he’s okay with getting rewritten.

I’m eager to get to work. Obviously, with Charlie and Corpse Bride, I’ve written musicals before, but this is the first time I’ll be able to incorporate songs into something approximating the real world. It’s also a chance to riff on musical cliches and conventions. Marc Shaiman (South Park, Hairspray) has signed on to write the songs, who seems ideally suited for the task. So here’s hoping.

One irony is that the director is Mark Waters, who’s made a good career for himself despite having turned down Go when it was offered to him. An unwritten rule of Hollywood is that eventually you work with everyone, so it’s nice to see that coming true.