First Answer
You are 3rd seat, both vulnerable.
What’s your call? With partner having passed, there is very little, perhaps zero, chance that we have a slam, so we like a four spade call. You are willing to compete to four spades almost no matter what happens, and you would like to avoid competitive bidding from the opponents. It’s not wrong to overcall one spade, but it’s been our experience that starting high in these sorts of situations tends to win matchpoints in the long run..
Say that you over call one spade. West passes, and partner bids two hearts. East passes.
What sort of hand do you expect partner to have, for this two heart call? What’s your call?
You will find a discussion of whether to overcall (or open) high or low in Chapter 5 of Real World Bridge 4: Competitive Bidding Part One.