There are states where judges never participate in plea negotiations (even if the rules might permit it), and there are states where judges do (sometimes regardless of the rules that prohibit judicial participation in plea bargaining).
Darryl K. Brown (University of Virginia School of Law) has posted What’s the Matter with Kansas — And Utah?: Explaining Judicial Interventions in Plea Bargaining (Forthcoming in Texas Law Review See Also, vol. 95 (February 2017)) on SSRN.
Here is the abstract:
This short piece suggests explanations for an apparent increase in a certain kind of “managerial judging” among state criminal court judges. In a recent study of ten states that authorize their trial court judges to play a role in the plea bargaining process, Nancy King and Ronald Wright found evidence that judges in eight states have indeed have become more involved in facilitating negotiated pleas. I suggest that an important likely reason for this development is that the eight states with active judges all have broad regimes of pretrial discovery. The two states in which judges have not moved into more active role — Kansas and Utah — are the two jurisdictions with much more limited pretrial discovery rules. Broad discovery was a key explanation the “managerial judging” that Resnik described in her classic account of federal civil litigation. Discovery — rather than simply rules authorizing a judicial role in the plea process — seems likely, for various reasons, to be an important factor in the somewhat analogous development of the judicial role in state criminal courts. I also consider the possible effects of other factors that make Kansas and Utah distinctive — they are the most rural states in the King-Wright study, and they have consistently had the lowest state incarceration rates.