Technique and Psychological approach are Different.
It is possible to train a person to defend themselves well. What it not easy to train for is the adrenaline you will experience in life threatening situation. This requires an actual threat and some real life experience. It is very difficult to simulate a shock. That being said, if a person is trained to automatically respond with a defensive manoeuvre when grabbed from behind they may find they automatically respond well in the heat of the moment, provided they were attached in that particular manner.
If Competitive Fighting Different To Self Defence
Yes, they are different, though there is also some overlap. For one thing, competitive fighting is always with somebody who is trained to a similar level to yourself; they will play be the rules and know how to attack and defend well. Street fighting is against an unknown assailant; they will be unpredictable and not follow any rules. If you know some martial arts, and if you can respond well in a crisis, you have a much better chance of avoiding serious injury. But the nature of street fighting is unpredictable.
Fights on film are choreographed and last for several minutes. A street fight may last all of 10 seconds. Self-defence teaches an individual to escape without harm, and possibly to hold off the attacker from another. The point is to avoid violence and injury.
Can Violence can Happen Anywhere, Anytime?
Some places are safer than others, and the type of threat occurs. You are unlikely to be mugged in hotel foyer, but there is a small chance of a terrorist attack. You are not subject to armed gangs in your own home, but domestic violence is painfully real for a few individuals. We are reasonably lucky in Australia; in some parts of the USA muggings occur openly on the street. Whether or not you come out of a confrontation physically injured it is mentally disturbing to know how vulnerable you really are.
Don’t ever fall into the trap of thinking you are always safe because you know some martial arts. You are better off for your training, but actively avoiding a confrontation, avoiding dark alleys and dangerous places, is always the wisest strategy. If you have no choice, then self-defence will at least give you some advantage, and you should suffer fewer injuries.