With lizardfolk, we get into the territory of generic humanoid monsters that are more than mere cannon fodder. They’re not sophisticated, but they are significantly tougher than goblins, kobolds and orcs. According to the Monster Manual flavor text, their most salient behavioral trait is their territoriality, followed by their generally acting like South Seas cannibals in a movie from the 1940s. On the flip side, the text does acknowledge that lizardfolk may occasionally form alliances with outsiders, but we’ll set that aside, since it’s not going to influence their combat tactics.
Lizardfolk, like orcs, are brutes: average Dexterity, high Strength and Constitution. They’re also proficient in Perception and Stealth, and they’re more or less amphibious—they can’t breathe underwater, but they can hold their breath for up to 15 minutes, and they can swim as fast as they can move on land.
Based on this information, the most likely lizardfolk encounter scenario will be with a group of scouts patrolling the outskirts of their territory. They’ll be alert to intruders—it’s why they’re out there. Once they notice intruders, they’ll start stalking them (either from cover to cover, if on land, or underwater, if in a swamp), until they’re close enough to attack. Then they’ll strike first, with surprise.