There's a few ways to analyze your case. To cut to the chase, in the end a lawsuit is about money. There's no point in suing someone only to be left with no recovery (it wastes your time and the time of your attorney). There are three things to consider:  recovery potential;  liability, and  injury.
 First, find out if your friend carries any insurance for personal liability - it should come with any renter's insurance policy. If there is no insurance, then I hope your friend is rich enough to pay you a substantial settlement (remember that bankruptcy will protect a lot of assets, and if you force him to go to bankruptcy - people will usually fight you tooth and nail to survive). No insurance = likely a no go, unless he's got substantial assets in his name and easy to find.
If he has insurance then the insurance will pay money only when the event was an accident (meaning not intentional). Assuming he was intentionally pushing you down the stairs - then the insurance will likely disclaim him and you will not get paid for your injuries from the insurance company. If he accidentally touched you and you were caused to fall down the stairs, then the insurance should pick up this case - he would have lawyers to defend him and money (up to the insurance limits) for settling your case against him. Make sure he reports this claim as soon as possible, or else the insurance will disclaim him for not telling them in time.
WARNING: Under no circumstances should you collude with your friend to commit insurance fraud. That is a very serious crime. Your case is what it is, it's not worth going to jail over.
 Next liability translates to a percentage of how wrong he was. If fault is 100% attributable to him then he pays 100% of the damages; if 50% liable, then he pays 50%, and so on and so forth.
 Your injuries translates to how much you can potentially get in compensation for your accident. The lighter your injuries, the less money you can get; more severe, more money. Obviously if you just got a little bruise, then its going to be very little; and if you get surgery, then there is more pain and suffering for you and as such you deserve more in compensation.
If your friend has substantial assets, and you sue under a theory of intentional torts, you may get something called punitive damages, meaning an amount that the court forces him to pay you as a punishment for his very bad behavior.
In the end, you should discuss this case in detail with an attorney. That's basically all you have to do. The lawyer will handle the rest of it.
If it was done on purpose, you should contact the police. If it was done negligently, you don't have to contact the police. In either case, you can demand payment fro the roomate for your hospital bills, lost wages, and pain and suffering. If the roomate agrees to pay, you should get the matter in writing and agree to a definite payment arrangement.
Also, if there is renters insurance in either of your names, you should contact the insurance company.
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