A lot of scripts that I've been reading lately have included character descriptions that go something like this:
JOHN DOE, think a Bruce Willis type of guy, approaches a woman who looks a lot like Diane Lane. In fact, let's call her DIANE.
I get the temptation to do this, considering the old axiom of "show, don't tell" but this isn't exactly what that means. There are several reasons NOT to do this, but here's the three main ones I can think of:
#3 -- Hollywood is a very small town. If you're lucky and someone high enough up the food chain is reading your script, they may actually know Bruce Willis. They might be his neighbor. They might think he's an asshole and, unable to get that image out of their head, will pass on the script just because of it. (Sorry Bruce, I'm sure you're a sweetheart, this is just a hypothetical.) Or the actor you choose as your role model was just in a stinker of a movie and the idea that this will do as poorly is hard for the reader to forget. Remember, the execs, agents and producers reading your script are HUMANS, with all the same foibles and flaws as the rest of us. Sure, they may LOVE Bruce Willis (who doesn't) and immediately want to keep reading but are you really ready to roll the dice like that on your script's future? If so, you should be living in Vegas.
#2 -- The reality is you don't really want people to THINK while reading your script, you want them to FEEL. Putting a name that they may not even know as a description just pulls the reader out of the script when all you want from page one is to pull them in. Directives like, "think a young Danny Kaye" are distracting unless you are a fairly AWESOME writer and can really slide that in there without it sounding klunky. Again, if you're willing to bet that you are that effen' good while still pitching your first or second script, you might want to rethink your career. Vegas baby, that's the place for you.
And the #1 reason NOT to use an actor as a character description?
It's perceived by many to be lazy writing. If you don't have the imagination or the vocabulary to describe your character in a way that a reader can instantly picture them, then the odds are heavily in favor of the script lacking an interesting character arc or storyline. It's not impossible, but it starts the reader thinking in that direction and it won't take much for them to give up on your script altogether. Studios want to know what character you're selling, not a vague generalization. What does a Bruce Willis type mean anyway? Fast talking from "Moonlighting" days, bigger than life action hero from "Die Hard" or creepy killer from "Day of the Jackal"?
The one exception: If you're writing a comedy and you want to use a comparison, like "JOHN DOE is so smoking hot he makes Edward the vampire look like Edward Scissorhands" or "DIANE is so blonde she makes Tori Spelling look like a brain surgeon". As long as it's true to the rest of the tone of your script, that's the one time name dropping can work in your favor.
It's fine for YOU to have Bruce Willis from "Die Hard" in mind as you write the scenes and dialog, but pull out the thesaurus and come up with a concise, vivid description to show everyone else your character's true identity.