Keith Richards thinks Sir Mick Jagger can be ''insane''.
The Rolling Stones guitarist - who has a difficult relationship with the singer - admires the frontman's self-discipline but sometimes thinks he can take it too far.
He said: ''I've more and more respect for Mick's resilience. Being the frontman always is a little bit of an isolated position, it's like being the captain of a Royal Naval ship. He's very disciplined, almost to the point of insanity sometimes.''
Meanwhile, the group's drummer Charlie Watts says Mick is a ''huge worrier'' and is concerned the 'Satisfaction' singer can be too anxious at times.
He told the Mail on Sunday's Review: ''Mick's a huge worrier. He's not like me and Keith. Sometimes I wonder if he worries too much.
''For me, it's a question of whether my hands hurt. With him, it's if his throat hurts, if he can't sing, he can't perform.''
However, the 68-year-old singer is the person the rest of the group go to with their difficulties as he is a good problem solver.
Charlie added: ''He's a very bright man, he's very aware. When we get on the road, people go to him more to sort things out. Thank goodness they've learned not to come to me.''