Drain it, before starting the car Is a perfect time to realise that the wrong fuel was put in, actually before even putting the key to on and allowing the pump to pull the fuel from the tank.
If you drain it all out nothing much will happen, especially if it's diesel in a petrol car, a colleague of mine had it done before, he actually drove away and the car sputtered to a halt just as he was joining the freeway.
Diesel was drained with the help of some tow guy, petrol put in but the car could not start, the guy towed it in gear with his tow bakkie, after what seemed like the biggest cloud of smoke I have ever seen the car came alive, and he has been driving it since then, a 2.0l Jetta.