Heh... well I was on fire once (ca 12 years ago) - proper flaming but not really full body. Seriously burnt ca 15% of skin on my body (if I remember my medical records correctly).
As far as pain... you get a couple of minutes of adrenaline filled "this doesn't hurts at all" after you are done extinguishing yourself. That is the time when you will still be thinking more or less clearly and not screaming - good idea to use it to to call on some one who will dial 911 (or what ever local equivalent is) and help you take off any tight fitting clothes, jewellery and watches you want to keep unclipped.
After that - when you more or less calm down you get to feel the burn (quite literally).
As far as pain... you get a couple of minutes of adrenaline filled "this doesn't hurts at all" after you are done extinguishing yourself. That is the time when you will still be thinking more or less clearly and not screaming - good idea to use it to to call on some one who will dial 911 (or what ever local equivalent is) and help you take off any tight fitting clothes, jewellery and watches you want to keep unclipped.
After that - when you more or less calm down you get to feel the burn (quite literally).