```python
from typing import List

def fill_between(entries: List[str], filler: str) -> List[str]:
    if len(entries) == 0:
        return []
    elif len(entries) == 1:
        return [entries[0]]
    
    result = [entries[0]] + [filler] * (len(entries)-1) + [entries[-1]]
    return result
```