Spoilers for The Reichenbach Fall
"Why me?" Mycroft demanded, when Sherlock told him his request.
"The joy of redemption, Mycroft," Sherlock replied, mockingly. "After all, you stitched me up with that journalist. You need this, you know, or you won't sleep at night."
"I could have arranged your entire demise, if you'd asked."
"Of course. You specialise in reusing corpses, don't you? But, as you see, I've managed that side myself, with some help from my friends. I'm giving you this minor role in my plan as a favour."
"For which you will expect repayment, naturally."
"Naturally. Dying is easy, staying dead requires more resources. So are you going to do it? Or do I cut you out of the loop entirely?"
"I don't see how we can make this work. John will recognise me."
"You'll be well disguised, and he'll be in shock. Henry Knight's mind created a monstrous hound, because it seemed more plausible than the truth. John won't believe it could be you, so he won't see you."
"I'm frankly not sure I can manage it."
"It's a simple enough task. A twelve-year-old could do it. In fact, I still have the scars from where you did do it to me."
"But I don't have the equipment."
"Oh, Mycroft, really. I'm sure your department's budget can run to buying you a BMX bike."