Very few canonical BX D&D monsters will automatically attack the PCs. Ghouls attack anything living. Zombies attack on sight. Goblins and NPC dwarves attack each without hesitation. Kobolds always attack gnomes. Tyrannosaurs will try to eat any creature man-sized or larger. But that's about it. (Displacer beasts always go after blink dogs. Frost and flame salamanders will go after each other. But those cases are rarely any of the party's business.)
Nearly every other kind of monster in the official BX rules is fully capable of exhibiting other behaviors at the beginning of an encounter besides attacking. Even berserkers don't mindlessly attack until they make an informed decision to mindlessly attack.
Additionally, unlike AD&D and its successors, the stat block in BX does not have a field for monster intelligence. Some entries specify the intelligence of a creature in the text, but many do not. There are a lot of monsters in the BX rules that I usually tend to assume are unintelligent only because I know their Monster Manual entry. The DM would be completely within the bounds of the rules as written to decide, for example, that an ochre jelly is smart enough to parley with. The text doesn't say anything either way. Even normal animals could talk in your campaign, if you wanted to give it a more fairy tale feel.
Now, consider the existence of the Monster Reactions table (page B24). Lots of monster encounters, especially wandering monsters, can be concluded without risking violence if the monster can be communicated with. Furthermore, a roll of 12 on 2d6 gives the result of "Enthusiastic friendship", giving the party a big incentive for talking to a lot of monsters. If you are going to explore a hellish, trap-laden underworld, having some sort of big, scary monster as your friend sounds great to me.
A 12 on 2d6 occurs only 1 throw in 36, but high charisma modifies the die roll. A Charisma of 13-17 gives a +1 on the roll, upping the odds of friendship to 1 in 12. Furthermore, the worst result on the chart, "Immediate attack", is no longer possible. An 18 Charisma allows for new monster buddies 1 in 6 encounters.
So the next time you spot a carrion crawler, try asking it how its day is going.
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