Rewriting an expression to call a generic interface's method

Through reflection I have an object that implements a generic interface, which I don't statically know its actual type. I want call one of the interfaces methods by manipulating an Expression. I've take a few attempts at this but I'm at a loss, any help would be greatly appreciated.

object anObject = /* Some reflection which gives IFoo<???>*/
MagicCall(anObject, x => x.MethodOnFoo("Bar"));
...
public static TReturn MagicCall<TReturn>(object obj, Expression<Func<IFoo<Object>>> expr)
{
    // Some manipulation of expr and obj
    return newExpr.Compile().Invoke(newObj);
}