But in a speech in Somerset on Sunday, the prime minister appeared to have taken this to heart and injected a bit more passion into his campaigning.
In a punchy speech, Cameron defended the decision to focus on risks posed to the economy by Labour and the SNP, insisting there was no "lack of drive" and saying claims he favoured the wealthy should be stuck "where the sun don't shine".
Cameron said those who accused him of "playing it a bit safe" ignored the central issue facing the electorate: "I have been Prime Minister these last five years. If people are saying to me we are putting too much emphasis on a strong and stable economy and in securing our future, I plead guilty."
He told activists in Somerset: "If you think economic security and stability don't matter, if you want to take a risk, go with the other guy, vote with the other man. By God he's got plenty of risks. He has found ideas that haven't worked anywhere in the world and he has put them in a book. It's called the Labour manifesto.
"I'm not going to put the British economy at risk, I'm not going to put the stability of Britain's families at risk, that is what is at stake in these last 11 days."
Cameron also spoke about his passion for leadership, saying Conservatives "don't always shout with the passion that some people would like, but don't mistake that for any lack of drive in changing this great country of ours and making it greater still.
"If you want political theatre - go to Hollywood," he told critics. "If you want political excitement maybe you could go to Greece, that's a very exciting country I'm told, every day government having to take money out of one part of government to pay people in another."