If you are running the Lord of the Crossing agenda, which says "During the third challenge you initiate each phase, each of your participating characters get +2 STR...", how would that work with Randyll? My guess is that you would declare attackers and kneel Randyll, then he would instantly get the +2 and stand.

Correct, he would instantly get the +2 STR upon participating in the challenge, and could react to that (provided he hadn't done so twice already) and stand.

Keep in mind that the triggering condition is only valid when he first becomes a participant in the challenge. For example, if you declare him as an attacker, you have to stand him right away, or not at all. You CANNOT just stand him anytime it is convenient during the challenge.

Thanks!