In the article linked above, Arnold tosses off a paragraph about how things that normally don't talk to players talk.
Well, that's super important. Everything or nearly so should talk in a fantasy game. I'm sure I can't recall the last time I played where someone couldn't speak to animals or plants or rocks or something. In a 4e game I played, someone figured out speak with dead along with the ability to speak with animals/insects allowed you to have access to effectively infinite sensors as long as you had enough access to mice or insect corpses.
The point is, if players can talk to stones, well, they are going to want to do it. The question is, how do you portray such creatures in game. This has an interesting intersectional tangent to my own beliefs, which is that pretty much all creatures have the same experience of life, even if they lack certain higher order functions. So, I'm asking a real question here, When the dogs can speak, what will they say?
Luckily, I'm preeety sure I can answer that question.
"Throw the ball? What are we doing now? Are we going somewhere? Did you say you were going to throw the ball? Is it time to eat? You're the best. I can get the ball for you. Or the stick. What's that over there? Here's the ball."
I mean, anyone who owns pets communicates with them all the time. Let's look at how to portray these things in game.
Speak With Animals
Happy, hedonistic, insecure. Seeking approval from pack leaders. Poor at making judgements or decisions. Can perform basic but insightful analysis of interactions. Unsure of answers. Can't count higher than two. Sees in black and white. Concerned with primarily relating scent information: Will talk about garbage, what people ate, what animals were nearby, in preference to what people are doing or saying.
As pets, but more rigid and disciplined. Can count to three. (One, Two, Three, Many). Speaks laconically and loudly, like a drill instructor. Calls all soldiers sir, doesn't take civilians seriously.
Selfish. Give no f%&*$. Narcissistic. Want to know why they should tell you anything. Acts superior, often bluffing. Haughty. Very self centered, when relating stories, every one is how about what happened affected them and their day. Secretly and urgently desires praise and attention.
Skittish. Nervous. Somewhat compulsive. When they've learned something, they can only relate it by rote, from the beginning each time. Everything is large or large and dangerous. Focused on the immediate world in front of them.
Focused on their needs. Totally convinced that they are part of the coolest organized crime group in the world and everyone should be terrified of them.
Speak with Plants
Totally unmotivated. Unconcerned with things. Not stupid, just lazy. Big plan of growing here, gonna keep it up. Speaks normally, but talks like a depressed person. ("That doesn't matter. I didn't pay attention, no one cares about that.")
Observant. Talks like sleezeball in a bar. Offers to grow on intimate places. Fond of damp, wet, organic matter.
Communal. They are all part of a drug-free commune, and are just not concerned with your petty concerns, man. Complains about being walked all over, but thinks that their marxist unity will elevate them.
Stoic. Slightly arrogant. Terrified of fire. Talks a lot about the sky and clouds. Completely uninterested in creatures on the ground.
Varies based on the season. Bright and happy in spring, full of excitement and promise. Talks about the future a lot. Becomes very slow and relaxed in summer, taking a long time to say anything. In the fall, becomes morose and vexing. Threatens and plays tricks on people. Whines and moans when not doing that. During winter, wails and moans constantly, is acutely depressed and apocalyptic.
They speak with the enthusiasm of a powerpuff girl.
Speak with stones
When new, these rocks are violent, and their voice is inconsistent and constantly changing, and they speak of change and revolution and tearing down the old order in fire and suffering. Older igneous rocks speak in a deep gravelly voice, that comes across as restrained power. They are jaded and have little insight into the world around them. Granite, especially cut granite, longs for the past, and believes things were better back then, and comes across as depressed and slightly lonely like old men.
Constantly contradicts self. Personality suddenly changes frequently. Disagrees with self. Confused. Clipped speech.
Slightly paranoid. Insightful but worried. Talks about instability and unpredictability. Cautions against depending on anything that it says. Believes the world is hostile. Somewhat pleased with itself and it's own traits and beauty, but then immediately falls back into paranoia.
Lots of different opinions, but all in the same voice from people who have identical experiences as them. Complains constantly about how nobody does any hard work and how they get taken advantage of by other people.
Narcissistic. Haughty. Spoiled. Throws tantrums.
Infantile wonder and amazement. "Whaaaaaaat?"
This article isn't meant to be a reference, but rather a starting point for thinking about the nature of things that normally don't speak. It's straightforward because the fact that the cat talks is the interesting thing. If, for whatever reason, that's expected, then it's no longer interesting. Players won't have thought about these characterizations; when they hear them, it will make sense. "Of course grass is a communal creature!" they will think. But if they are expecting it, then feel free to play against type to break expectations, once the thing talking is no longer the interesting thing.
Also, seriously, speak with rocks? Whoever invented that one was a jerk. Everything is made out of rocks. It's as difficult to manage as Psychometry is in game!