Jim Mallinder has urged his Saints players not to show Castres too much respect when the two sides go head to head in Toulouse on Friday night.
Mallinder wants his men to play to their maximum as they bid to secure a second win in as many Heineken Cup games this season.
Saints were made to suffer early on against Glasgow Warriors at Franklin’s Gardens on Sunday as they went 15-0 down inside 27 minutes.
But that only served to spark them into life and they stepped on the gas to seal a bonus-point 24-15 win.
Mallinder wants more of the same against Castres on Friday, but he knows Saints can’t afford to stand off the Top 14 side.
“We need to step up,” said the director of rugby. “We know that when we’re on the ball and on the money, we’re a very good side.
“When we take our foot off a little bit and relax and give them a little bit of respect, we’re not very good.
“We’ve got to make sure our intensity is right and that our tactics and team play are right. If we do that, we can get that elusive win.”
Friday’s game will be a step into the unknown for Saints as they take on Castres away from the French side’s regular home.
The game has been moved from the Stade Pierre Antoine to the Stade Ernest Wallon, but Mallinder is not sure the venue switch will have too much of an impact.
He said: “It’s hard to say whether it will make a difference.
“We’ve not played recently in that stadium so we don’t know how they’ll take to that stadium, who’ll be there and the amount so that’s probably a bit of an unknown, even though they did play there a couple of times last season in the Heineken Cup.
“We can’t worry too much about that. What we can worry about is how we play and our performance and we’ve got to make sure we get that bit right.”