The most direct way to get a reference to another component is to use the GetComponent method.

using UnityEngine;

public sealed class GetComponentExample : MonoBehaviour
{
    private void FixedUpdate()
    {
        var rigidbody = GetComponent<Rigidbody>();
        rigidbody.AddForce(Vector3.up * 10);
    }
}

The above example will search for a Rigidbody component attached to the same GameObject as this GetComponentExample component. Then it applies some upwards force to that Rigidbody.

Advantages

Simple - When you need something, you just get it where other approaches require a bit more setup.
Short - You could even write it on a single line as GetComponent<Rigidbody>().AddForce(Vector3.up * 10);.

Disadvantages

Summary	Description	Solution
Slow	Searching for another component has a notable performance cost which applies every frame with this approach.	Get Component Cached is the direct solution, but all other approaches solve this problem.
Unsafe	Adding the `GetComponentExample` component to an object in the Unity Editor gives no indication that it also needs a `Rigidbody` component attached to the same object. You will only find out when you try to run the game and it spams the Console window with a `MissingComponentException` every frame.	Require Component helps, but Serialized Field is better.
Becomes very verbose when repeated	If you need to access the `Rigidbody` in multiple places, repeating the `GetComponent` call for each one will take up a lot of space.	All other approaches take a bit more code to set up a field, but then the field can be easily used throughout the whole script.
Inflexible	The `GetComponentExample` needs to be on the same `GameObject` as the `Rigidbody` unless you edit the script.	Serialized Field.

Get Component

Advantages

Disadvantages