So you've spent two weeks creating the structure and outline of your screenplay. You're happy with how the story flows from a high-level point of view and you're ready to start fleshing out your scenes.
Before you begin, you spend some time thinking of character names. It's a detective story so you feel that something like "Dean" is a good name. Yes, "Dean McCormick", great. Sorted. You then start thinking of your antagonist's name and for whatever reason "Gene Davis" pops into your head and it sticks.
Good guy: Dean McCormick
Bad guy: Gene Davis
As a whole name, and as read, they read completely different, but whilst Dean and Gene look sufficiently different in your screenplay, on screen they'll be very hard to distinguish in dialogue. It doesn't take a great leap of imagination to work out that if another character is referencing Dean or Gene in their dialogue, it could be confusing for the viewer.
However, after saying that... You may actually WANT this to happen as a device in your screenplay - to cause the very confusion it could cause, but be careful of this device. If done incorrectly it could just end up falling flat on its face.
Admittedly, there aren't many names that rhyme that look different but I have seen it done in spec screenplays online (Dean and Gene in fact) and by that time the writer has grown quite attached to his or her character names and finds it hard to change them.
So, as a rule of thumb avoid that trap by saying your character names out loud before you commit to naming your darlings.