The function swaps the first and last characters, as well as the second and second last characters of the input string.

[CODE]:
```python
def swap_characters(s):
    s = list(s)
    s[0], s[-1] = s[-1], s[0]

    if len(s) > 2:
        s[1], s[-2] = s[-2], s[1]

    return ''.join(s)
```

Let's try one more test to verify this description.

COMMAND: PYTHON(./temp/function.py "This is a test" "abcdefghijklm")