Following a 1-0 World Cup qualifying win against minnows Andorra in Barcelona, England were defeated 3-1 by Senegal at the City Ground in a friendly. It marked a first ever defeat to an African nation for England.
Walcott told the Daily Mail: "The Germans are so serious and sometimes we're not as serious.
"That's a shift we need to make and you could sense that against Senegal the other day (in the seniors).
"We didn't seem serious because it's a friendly and everyone wants to go on holiday. We need to start being more serious, particularly at international level."
Walcott also remarked: "I think it's really valuable and important to be in good habits and to actually grab that opportunity and think: actually, this is really important.
"I think if you go into it half-heartedly, anything you do, maybe you're not going to fail, but you're not going to really enjoy it, right?
"I understand talking on the field is a cliche, but I actually get the sense that if you want to be in the first team now, in Thomas Tuchel's team, going to the World Cup, you need a little bit more, like really going into it. It's very different, I feel.
"It's the full package now that they're looking at."