Good last point, actually. I think that's perfectly true, of course because:
1. A good performer who has shown they are a good performer never stops being one and
2. You may be exactly right for a different type of part that comes up in a different project - at which point, your own style becomes a benefit.
So yes, there's no such thing as a totally useless audition!
Interestingly, although there's a bit of a distinction between those who screen audition 'on spec' and those who plot everything out in advance carefully emerging here, I'd say the majority of comments actually agree upon a few essential things:
That you should try and get a script for any such audition, and really read it through well;
That it's good to let the lines 'play around' in your brain, try them out in multiple different ways, and possibly that learning them *isn't* the most important thing, but *getting used* to them is...
That (however you do it - with a partner or alone), it's good to examine the cue lines in detail as well, so you can think about how your reactions to them feel...
That, on the day, to a certain extent, it's better to go with impulse and instinct (built on a base of solid preparation) rather than to have religiously worked everything out to the nth degree - that way, the auditioners get to see you, and your spontaneous ideas...
All good thoughts. Frankly (and at the risk of derailing this thread a bit!), I often find that the hardest aspect of screen audition(for me) is physical, rather than mental - where to put your eyes, what the best position to hold a script you're reading from is, how long a pause you can justifiably leave before you give a reaction to a cue etc. I can find that if I'm worrying about all that too much, it gets in the way of the actual acting...but I personally think there's much more to be concerned with at any one time in a screen audition than in a stage one - pitching things vocally well, keeping good reactions going whilst sight - reading, avoiding too much gaze into the camera, listening closely to your scene partners, even positioning improvised scenes correctly for film usage, rather than stage usage. All complicated stuff, and I guess many of us who've yet to work on something really big - budget and/or hyper - intensive film - wise can get a little bamboozled by it all sometimes!