Detective Loach (Dick Bakalyan)
Loach assists Lieutenant Escobar with the official investigation into Hollis's murder. He encounters Jake when Jake goes to the reservoir (which turns out to be the scene of that murder).
Duffy (Bruce Glover)
Duffy is Jake's assistant, along with Walsh. He attends Jake as he investigates Hollis and the mystery of his murder; for instance, he accompanies Jake on a rowboat in Echo Park as they trail Hollis and the woman who they think is his mistress (but is actually Katherine Cross).
Maid (Beulah Quo)
This is Evelyn's maid. Katherine's apparently staying at her house, which is where Jake confronts Evelyn about Katherine, forcing her to admit that Katherine is her daughter and her sister.
Councilman (Noble Willingham)
At the city council meeting about building a new dam, this guy takes the mic for a second to ask to hear from Hollis Mulwray again…and that's it.
Emma Dill (Cecil Elliott)
Jake meets Emma Dill when he goes to the retirement home that Noah Cross is using to cover up his land and water grab. Emma Dill's listed as one of the landowners (of land that's actually Cross's), and Jake tells her she's a rich woman.
Emma denies it, totally unaware of the land she's supposed to own. But she ends up showing Jake that Noah Cross's Albacore Club is a major sponsor of the retirement home.
Mr. Palmer (John Rogers)
Palmer runs the retirement home funded by the Albacore Club, and is apparently in on Noah Cross's scheme. When Jake starts snooping around, Palmer contacts Claude Mulvihill—who Jake ends up pounding in a fight.
Morty the Mortician (Charles Knapp)
Morty works at the morgue, and gets a great line about Hollis Mulwray's death:
MORTY: Can you believe it? We're in the middle of a drought, and the water commissioner drowns. Only in L.A.
He also tells Jake that one of the dead drunks in the morgue drowned in the L.A. River—but Jake has already been to the river, and he knows that it's dry due to the drought. He realizes that someone's stealing water by releasing it through the river late at night (or rather, the water that comes down the river is run off from the water they're stealing).
Gardener (Jerry Fujikawa)
Evelyn's gardener doesn't realize it, but he supplies a very important clue. He tells Jake that the water in the pond in the back of Evelyn's house is "bad for the grass"—because it's saltwater. Jake realizes that this is why Hollis Mulwray had saltwater in his lungs. His murderer (who turns out to be Noah Cross) drowned Hollis in this pond.
Boy on Horseback (Claudio Martinez)
This boy on a horse tells Jake that water is being released at night through the L.A. River, and that it comes at different times. Hollis apparently asked the boy a similar question earlier, when Jake was spying on him with his binoculars. This kid's a classic information tool.
Mulwray's Secretary (Fritzi Burr)
While Jake waits around to meet with Russ Yelburton at the water department, Mulwray's secretary tells him that Hollis Mulwray felt that the public should own the water supply while his partner, Noah Cross, wanted to continue owning it privately.
This helps explains Noah's motive for trying to gain control of the water supply—he wants to make it his again and control the development of the city.