Since all characters have to declare thier actions before determining initiative, there are two scenarios...
First Scenario: Prior to the current round, the tank found out the caster was going to be attacked, and now needs to know how to resolve declarations and actions.
Second Scenario: During the current round, the tank sees the caster trigger the trap/monster and now needs to cancel his previsously declared actions in order to lend a hand.
Regardless of which scenario you're looking at, the basic question now is...can the tank make it in time to help and what happens if he can......
FIRST SCENARIO--caster has to "beat" the attack on the caster by acting before the attack is launched. If the trap/monster attacks in the Snap Action phase, then the only way to beat it is to declare a Snap Action and have a higher initiative. If the attack will occur in the Normal Action phase, then the tank could declare either a Snap Action (with higher or lower initiative) or a Normal action move manuever (with a higher initiative than the attacker). Assuming the tank can fairly easily put himself within distance to help, you could do a Conflicting Actions test (tank vs trap/monster-page 53 RMFRP), but I would suggest that the tank be required to make a Medium Move Maneuver roll (with applicable mods such as -20 for snap action, etc) with the result being the amount of the trap/monster attack that he deflects upon himself. For instance, if the caster has triggered a spear trap (OB 80), the tank could declare a snap action "move and defend the caster". Then, the tank rolls on the manuever table and gets a result of "57". The GM now makes two attack rolls for the spear trap, one against the tank (at OB 57) and one against the caster at (OB 23)(with an additional modifier for cover provided by the tank). So maybe the tank deflected the spear enough to save the caster, or maybe he didn't (I'd use a single attack roll and apply it to both attacks).
SECOND SCENARIO--(Please remember this takes longer to read than to play). Here we are assuming the trap/monster attack was triggered during the current round, but after all action declarations. So, the tank has to cancel his declared actions in order to lend a hand to a new situation. There are rules for this (page 54 RMFRP--tank actions all resolved during Deliberate action phase) but I don't like them and have my own House Rule which I'll apply to this scenario (assuming the trap/monster attack is triggered during the Snap Action phase): Since the tank had initiative,his declared Snap Actions are already done and he cannot intercept a Snap Action attack against the caster. If he didn't declare a Snap Action, or if the trap/monster attack will occur in a later phase, the tank could react by declaring "Cancel my actions and protect the caster!". Whenever a character cancels declared actions, I require them to make a Medium Move Maneuver to determine how much of thier remaining activity they've "squandered". Then, I require them to re-roll thier own initiative (yes, this means their initiative could improve by doing something unexpected, but usually at a high cost of activity). Notice that high AT's make this even more difficult because of thier intrinsic MMP's. So the tank rolls a Medium Maneuver and gets a "55". This is the % activity left to him to use that round. Now he re-rolls initiative (hoping he didn't lose the initiative to the trap/monster) and still beats the trap. This basically brings us back to the first scenario, where we either do a Conflicting Actions test or the tank determines how much of the trap/monster OB is deflected upon himself.
It all plays out very quckly, really.