"When two or more persons have collaborated in the commission of a criminal offense and are later apprehended for questioning, there is usually a nagging fear on the part of each participant that one of them will "talk." Individually, each of them may feel confident of his own ability to evade detection and to avoid confessing, but they generally do not experience a comparable degree of confidence with regard to the co-offender's ability or even willingness to do so. Uppermost in their minds is the possibility that one of them will confess in an effort to obtain special consideration.”