Northampton head to the Stade Pierre-Antoine, hoping to kick off with a win in Pool 1, which also includes RaboDirect PRO12 sides Leinster and Ospreys.
Jim Mallinder’s men will be buoyed by a strong start to the domestic season, having won three of their first five Premiership games, also picking up a draw at Leicester last weekend.
But attack coach King feels Saints will need to up their intensity levels if they are to taste success in Europe this weekend and beyond.
“We want to get off to a good start, but by losing on the first weekend doesn’t determine anything: it’s how you do in the six rounds that counts,” he said.
“We want to go to Castres in a positive mindset, we want to play and want to reproduce the form we’ve shown in the first five games of the season.
“But we understand it’s a big step up from the Premiership.”
King was part of the Clermont Auvergne coaching team last season, helping Les Jaunards to the final of the Heineken Cup.
They eventually lost to Toulon in May’s Dublin showpiece, but King has received an education in what it takes to win the tournament.
“It’s the consistency to play at the top level over a period of months,” said the former Wasps and England fly-half.
“There are games in October, December, January and then hopefully knock-out stuff in April and May so your team and your squad has to stay fit and on form because it’s a short tournament in terms of games but each game is a step up from normal league rugby.
“It’s a great tournament for the players, coaches and fans and I hope they sort something out for next year because it is a wonderful tournament.”