Eoin Reddan sees Castres scrum-half Rory Kockott as a "danger" to Leinster.
"There is not a second he spends snapping or trying to talk rubbish," said the Irish half-back (above).
"He knows what he's doing. He's always looking. And sometimes they are the most dangerous guys, in my book.
"They're constantly looking around the pitch and you're trying to guess what they're thinking, where are they going to go next.
"They're the guys you need to worry about, not the ones who are holding you, jumping on you. He's looking for opportunities."
He says supporters were "rightly" concerned about Leinster's form going into Europe.
"There was definitely an edge about us last week that probably allowed us to get out of a hole. It was a tough hole to be in, to be honest.
"You start the game at 0-0 and then all of a sudden you are 12 points down with five minutes to go in the half.
"So, I think we stuck at our task quite well, even though we didn't necessarily need to give ourselves that task to do.
"We had to actually stay calm and build a bit of pressure and tempo, field position, be smart, and put us in a position where we won the game".
Another bad start against Wasps to go beside the bad start to the season.
It was there in Castres last season. And Reddan's reaction then?
"Oh my God," he recalled.
"It felt like we were 14-0 down after about 20 seconds, you know? In a game that we had to win.
"They are a team I know well because I went down there with Wasps in 2008 where we had to win to qualify and they were out by a mile. We were a fairly decent side and they ended up beating us pretty well.
"They buck the French trend in that every time you play them you will have people writing about them and asking 'are they interested?' because it's a French thing.
"In my experience, I have never played down there where they haven't been anything but 110% committed".