When the serpent in the garden spoke to Eve and people perceive animals talking to them what's going on? Human psychology says, "The disassociative mind can sometimes transfer auditory hallucinations onto objects like animals or toys which can help a person localize an experience [whatever that means]."
But the DM-5 Psychology Manual says group hallucinations are impossible. So when the Apostles saw Jesus, touched Him and talked with Him in various group settings after His death and resurrection, this definition of human psychology breaks down. Therefore, it stands to reason when a human being hears an animal speak it is potentially possible some evil spirit is communicating through that animal to transmit auditory even visual messages.
These are usually demons not fallen angels, because demons are disembodied spirits and seek to inhabit animals even humans. And the messages they convey can often seem harmless even helpful or loving, but their aim is always to deceive and harm. These messages cannot come from loved ones who have gone to sleep because they are still sleeping. Resurrection has not taken place yet. 1 Thess. 4.14-18 says be comforted by this fact as we will be resurrected together.
Fallen angels are limited in their function like angels. Man will eventually rule over angels. Angels take up the prayers of the saints (Rev. 5.9) but, of course, they don't possess people as that would be unholy. But it is demonic spirits that actually possess a person. Fallen angels had sex with daughters of men to create giants. They did this by somehow manipulating the genetic code when two humans have sex. That's why God gets Israel to wipe them out, because they warp the genetic code. This matter was taken care of in the past in Noah's day with the local flood.
The spirit initially is from without as spirits can only work from the outside to the inside; whereas God works from inner to outer. As the spirit enters, the message takes place in the intuition, communion and conscience of our spirit and moves from the spirit to the outward senses such as hearing and seeing through man's soul. So just because we can't prove theses experiences are true when they occur on an individual basis, Jesus was seen in various group settings so we know it is possible a person can have an individual real life experience with a demonic or evil spirit or an angel even with Jesus when they are alone.
For example, Paul saw and heard Jesus on the road to Damascus, but only he could see Jesus. As Acts and Paul's epistles report, those with him saw the light and heard the voice, but they did not see Jesus. In reaction they fell to the ground with Paul because of the awesome light.
That's the best way I know how to explain these type of experiences in Scripture and continue to this day. You don't have to be a Christian to have these experiences as evil spirits and demons will enter any vulnerable soul. Though Christians have greater protection, of course, with the full armor of God. As for angels their manifestation, obviously, would be greater in Christians since we have the Holy Spirit and nobody else does.