Since memoir is written in first person, from your point of view, you can only directly describe your own thoughts and sensations. When you write scenes that include other people, you are limited to writing what you can observe or what they tell you. For example, it’s fine to write “The air was precisely skin temperature, and I was tempted to run naked to celebrate the return of summer, but I was afraid old man Jones would peak through a knothole in the fence, and I didn’t want to be sullied by his eyes.”
You can write that about yourself, because you know your own thoughts. You can’t tell us that Janna was tempted to run naked unless Janna told you that herself. In that case, dialog would be a good way to put those words in her mouth. Let her tell her part of the story herself. For example,
“What a glorious day!” I said, raising my arms to the sun and lifting my face.
“Oh yes! It’s perfect! The air feels like it isn’t even there. I’m tempted to peel off my clothes and run naked, but I’d die if old man Jones looked through that knothole. I’ll bet he does when he hears us out here. I don’t want that old lech looking at me even with my clothes on!” Janna replied with a glaring glance toward the fence.
There are other ways you can maintain the integrity of your story if you can’t cite first hand knowledge of someone else’s experience or thoughts. For example, in her memoir, Sixtyfive Roses: A Sister’s Memoir, Heather Summerhayes Cariou writes about a home visit the doctor made to her sister: “He took Pam’s temperature. Gently, he folded back her nightie, slowly moving his stethoscope over her emaciated chest and across her bony white back. I wondered if it felt cold.”
Heather tells us what she saw the doctor do, then she tells us what she didn’t know. That’s powerful. She maintains her credibility. We don’t actually know. If it had been in the doctor’s pocket, maybe it wasn’t. Heather wondered, and so do we.
In other circumstances, you can speculate, “He must have felt like he’d just walked on water.” Or you can substitute your reaction, for example, “If I’d been in her shoes, I would have screamed bloody murder. It just didn’t seem fair.”
Remember, this is your story. Tell it like you saw it, felt it, heard it, smelled or thought about it, but don’t speak for anyone else. Always keep the boundaries clear between your thoughts and perceptions and those of your supporting cast.