Overcalls have more offense and less defense than openers. Ironically, this means that your responses should be about the same for overcalls as for openers, for different reasons.
Your partner's overcall shows about eight points as you said. Together with your six, that means 14 for your side and 26 points for your opponents. They probably have an eight card fit, and the ability to make four clubs, four diamonds or four hearts. If their suit is hearts, they have game; otherwise they have a part score. If that's the case, you don't mind "going down" by say, one or even two tricks, as much as if they didn't have a four level contract. Ironically, your weakness (and oppoents' strength) causes you to bid more aggressively.
Your partner's bid showed five spades (as in an opener) headed (probably) by three honors. Between the two of you, you should have five "natural" spade tricks, one ruff, and one to two side tricks, for a total of seven or eight. On offense, your hands are worth more than 14 points because your spades are solid. If you raise to two, you might make it, and worst LIKELY case, you'll go down one. Given this fact, you probably won't be doubled, and "down one" is better than letting the opponents have a part score in three or four diamonds or clubs.
Because of the opponents' potential for a heart GAME, some experts say that you should go to THREE spades if not vulnerable. That's down one or two, 100-300 points if doubled. Down one (-100) beats their likely part score, and down two (-300) beats their possible game. You can be more aggressive if your opponents are timid doublers because the penalties will be less.