For long term (more than a day) rentals, they will block it off your credit card (pre-authorise). Small time operators have higher accident insurance excess (Qr 2500) but lower rents. Big names like Hertz, Avis etc have lower excess (approx Qr. 1000) BUT higher rent cost. This money gets cut from your account if you are in an accident and don't have a police report, or the report states it was your fault.

If you are renting at the airport (and you're not a permanent resident of Qatar) it is possible they will ask u the money upfront.