The Priestly Passover legislation of Exodus 12:1-11 not only stipulates the preparation of the sacrificial animal prior to its slaughter (#111), but also how it is to be cooked and eaten. “Do not eat any of it raw or cooked in water, but fire-roasted” (12:9).
Yet this is not at all what is relayed in the Passover legislation found in Deuteronomy 16:1-8. There we are told that the paschal lamb or cattle is to be cooked, and what is implied is cooked in water, that is boiled.
Boiling was the typical and normative practice by which sacrificial animals were prepared and eaten (Lev 6:21-28, 8:31; Num 6:19, 1 Sam 2:13-15, etc.). The strict decree to fire-roast the paschal lamb in the Priestly legislation, where elsewhere in the book of Leviticus, also penned by P, meat is boiled, might suggest that fire-roasted represents an older tradition. In fact, it is the language of the whole burnt offering in Leviticus 1; but none of that meat is eaten.
At any rate, the non-cultic language of the Deuteronomic version is just one example of its secularization of the ritual. While the Priestly author is concerned with the proper ritual preparation of the paschal lamb and its consumption, the Deuteronomic author is more concerned about where this meal gets eaten: “at the place where Yahweh shall chose to tent his name.” See forthcoming #118.