OK, I'll throw in some thoughts here since I've had a few interesting experiences that are relevant.

If you are renting, make sure you like the car when you drive it, or you and others may suffer the consequences! I co-drove a friend's Porsche 944 ITS car that wasn't super fast but was a very predictable nice handling car no matter what the fuel state or condition of the tires, etc. and that's a car I could drive all day and not have any problems with. I also co-drove a ITB Golf that was again not super fast but well prepared and easy to drive, and again I had no problems and could have done stints in the car for days on end.

Most recently I co-drove a car that I did not think was as well prepared as I had expected, and I was rather uncomfortable driving it. The crew chief and I never really got on the same page with our communication about setup, etc. and I spun the car on my own multiple times in practice. I may have been running into my talent limitations as far as driving, but I just couldn't really come to grips with the car. At the time I thought about possibly bowing out of the whole deal and letting them keep my rental fee, and in retrospect that's probably what I should have done. In the race I was partly to blame for a crash that totaled two cars, and it might have been avoided had I not driven the car in the race. If I had withdrawn from the driving squad the team might have had to find another driver, or maybe the other rental drivers would have gotten more seat time for free... In any case I understood my obligation after the accident and ended up paying for a destoyed car.